Calibration of magnetometers

Abstract

A method for performing calibration of magnetometers is provided. In some embodiments, the method involves obtaining a sequence of gyroscope measurements from one or more gyroscopes and a sequence of magnetometer measurements from one or more magnetometers. In some embodiments, the method involves determining a sequence of angular velocity estimates based on the sequence of gyroscope measurements. In some embodiments, the method involves determining a first estimate of a derivative of an external magnetic field based on the sequence of magnetometer measurements. In some embodiments, the method involves determining a second estimate of the derivative of the external magnetic field based on the sequence of angular velocity estimates. In some embodiments, the method involves identifying magnetometer calibration constants based on a difference between the first estimate of the derivative and the second estimate of the derivative.

1 . A method for calibrating magnetometers for motion tracking devices, comprising:
 obtaining a sequence of gyroscope measurements from one or more gyroscopes and a sequence of magnetometer measurements from one or more magnetometers;   determining a sequence of angular velocity estimates based on the sequence of gyroscope measurements;   determining a first estimate of a derivative of an external magnetic field based on the sequence of magnetometer measurements;   determining a second estimate of the derivative of the external magnetic field based on the sequence of angular velocity estimates; and   identifying magnetometer calibration constants based on a difference between the first estimate of the derivative of the external magnetic field and the second estimate of the derivative of the external magnetic field, wherein the identified magnetometer calibration constants are usable for calibrating an orientation tracking system that comprises the one or more gyroscopes and the one or more magnetometers.   
     
     
         2 . The method of  claim 1 , wherein the sequence of gyroscope measurements and the sequence of magnetometer measurements are obtained during movement of the orientation tracking system. 
     
     
         3 . The method of  claim 1 , wherein identifying the magnetometer calibration constants comprises identifying the magnetometer calibration constants that minimize the difference between the first estimate of the derivative of the external magnetic field and the second estimate of the derivative of the external magnetic field. 
     
     
         4 . The method of  claim 1 , wherein determining the first estimate of the derivative of the external magnetic field comprises determining a difference between at least two magnetometer measurements of the sequence of magnetometer measurements. 
     
     
         5 . The method of  claim 4 , wherein the at least two magnetometer measurements span a time duration within a range of about 80 milliseconds-120 milliseconds. 
     
     
         6 . The method of  claim 1 , wherein determining the second estimate of the derivative of the external magnetic field comprises determining a product of a representation of a change in the sequence of angular velocity estimates and a representation of the sequence of magnetometer measurements. 
     
     
         7 . The method of  claim 1 , wherein identifying the magnetometer calibration constants comprises identifying an eigenvector corresponding to a smallest eigenvalue of a matrix representative of the difference between the first estimate of the derivative of the external magnetic field and the second estimate of the derivative of the external magnetic field, and wherein elements of the eigenvector correspond to the magnetometer calibration constants. 
     
     
         8 . The method of  claim 7 , further comprising:
 determining a second smallest eigenvalue of the matrix representative of the difference between the first estimate of the derivative of the external magnetic field and the second estimate of the derivative of the external magnetic field; and   determining that a second sequence of gyroscope measurements and a second sequence of magnetometer measurements are to be obtained based on a ratio of the smallest eigenvalue to the second smallest eigenvalue.   
     
     
         9 . The method of  claim 1 , wherein the sequence of angular velocity estimates is determined using the sequence of gyroscope measurements adjusted using gyroscope calibration constants, and wherein the gyroscope calibration constants are determined based on gyroscope measurements obtained while the orientation tracking system is held stationary. 
     
     
         10 . The method of  claim 1 , wherein the sequence of angulator velocity estimates is determined using an initial set of gyroscope calibration constants, and wherein the method further comprises:
 determining a calibration error associated with the magnetometer calibration constants identified using the initial set of gyroscope calibration constants;   updating the initial set of gyroscope calibration constants based on the calibration error to generate an updated set of gyroscope calibration constants; and   determining updated magnetometer calibration constants using the updated set of gyroscope calibration constants.   
     
     
         11 . The method of  claim 10 , wherein the updated set of gyroscope calibration constants are determined using gradient descent. 
     
     
         12 . The method of  claim 1 , wherein the one or more gyroscopes and the one or more magnetometers are disposed in or on headphones or a head-mounted display. 
     
     
         13 . The method of  claim 12 , further comprising:
 utilizing the magnetometer calibration constants to determine a spatial orientation of the headphones; and   modifying spatial characteristics of audio presented via the headphones based at least in part on the spatial orientation of the headphones.   
     
     
         14 . The method of  claim 13 , wherein the spatial characteristics of the audio are modified in near real-time based on a continuous series of determinations of the spatial orientation of the headphones. 
     
     
         15 . The method of  claim 1 , wherein the one or more gyroscopes are configured to measure rotation along three axes, and the one or more magnetometers are configured to measure magnetic field along the three axes. 
     
     
         16 . The method of  claim 1 , further comprising:
 determining a calibration error associated with the identified magnetometer calibration constants; and   causing an indication of the calibration error to be presented in a user interface.   
     
     
         17 . The method of  claim 16 , wherein the calibration error is determined based on a ratio of a smallest eigenvalue to a second smallest eigenvalue associated with a matrix representative of the difference between the first estimate of the derivative of the external magnetic field and the second estimate of the derivative of the external magnetic field. 
     
     
         18 . The method of  claim 16 , further comprising:
 obtaining a second sequence of gyroscope measurements and a second sequence of magnetometer measurements;   identifying updated magnetometer calibration constants associated with the second sequence of gyroscope measurements and the second sequence of magnetometer measurements;   determining a second calibration error associated with the updated magnetometer calibration constants; and   updating the user interface with an indication of the second calibration error.   
     
     
         19 . An apparatus configured for implementing the method of  claim 1 . 
     
     
         20 . One or more non-transitory media having software stored thereon, the software including instructions for controlling one or more devices to perform the method of  claim 1 .